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Title of the thesis: Biocolorants for Engineering Materials

Thesis defender:  Juha Jordan
Opponent: Prof. Tero-Petri Ruoko, Tampere University, Finland
Custos: Prof. Jaana Vapaavuori, Aalto University School of Chemical Engineering

Although colors surround us everywhere, most of them are still created with synthetic colorants derived from fossil resources. Biobased colorants, produced from renewable raw materials, provide a more sustainable alternative to synthetic colorants. To date, however, biocolorant research has focused mainly on textiles, leaving their use in engineering materials relatively unexplored.

This doctoral thesis investigated how biobased colorants can be applied practically and durably to engineering materials. The thesis consists of three studies, two of which examine the use of indigo from Isatis tinctoria in coloring wood coatings and polypropylene. In addition to indigo, the third study investigates also the use of madder (Rubia tinctorum), weld (Reseda luteola), kraft lignin, and biochar in polylactic acid coloration. Accompanying the three published studies, this thesis contains a previously unpublished work investigating biocoloration of anodized aluminum with madder and weld extracts. In all studies, durability of the colored materials was evaluated by exposing them to artificial weathering and Xenon light, followed by detailed colorimetric and spectral analysis. 

The results demonstrate that in many cases, biobased pigments can reach performance comparable to conventional alternatives. For instance, indigo provided wood coatings with weather resistance on par with mineral pigments, and its purification improved both color intensity and lightfastness in polypropylene. Kraft lignin proved especially promising for polylactic acid, not only enhancing resistance to photodegradation but also improving mechanical strength. Furthermore, anodized aluminum surfaces were successfully colored with madder and weld, producing thin, evenly coated metallic hues. 

The thesis concludes that biobased colorants can be effectively applied in engineering materials while ensuring long-term durability. These findings open up new opportunities for more sustainable material production, offering potential applications in plastics, coatings, and even metals.

Keywords: biocolorants, engineering materials

Thesis available for public display 7 days prior to the defence at .